This invention relates to photoflash lamps and, more particularly to flashlamps of the type containing primer material ignited by a high voltage pulse.
Such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
An improved ignition structure, which provides excellent lamp reliability and substantial economies and ease of automated manufacture, is the so-called fritted lead construction described in U.S. Pat. No. 4,059,389, of Donald E. Armstrong et al. This is a beadless ignition structure comprising a pair of spaced-apart lead-in wires with spherically shaped terminations, a glass frit coating over the lead-in wires, and a coating of primer material over the frit-coated terminations. The primer may bridge the wire terminations or comprise separate spaced apart coatings on the respective terminations, with the filamentary combustible (shredded foil) being in contact with both terminations to provide a conducting path there between. A primer composition is described which comprises about 99.0 percent by weight of zirconium powder and 1.0 percent by weight cellulose nitrate on a dried basis.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, for example, from a piezoelectric crystal in a camera) across the lamp lead-in wires. The dielectric primer coatings within the lamp then break down electrically and ignite; the resulting deflagration, in turn, ignites the shredded combustible which burns actinically.
Normal lamp-to-lamp variations as well as varying degrees of intimacy of contact between primered leads and shredded combustible gives rise to a wide range of lamp firing or breakdown voltages. So long as the maximum lamp breakdown voltage is below the output pulse voltage of the camera the lamps will operate reliably. The maintaining of a suitable control of the upper limit of lamp breakdown voltage in automated lamp production has not been a problem.
Prior to use of the present invention, some lamps tended to have very low breakdown voltages; values as low as 50 volts sometimes being encountered. Such lamps tended to be undesirably sensitive toward inadvertent ignition by stray electrostatic charges both during manufacture and use. Also, there have now appeared on the market cameras that give voltages to the flash socket at times other than during actual picture taking. These spurious camera pulses are generally below 100 volts but can, however, give rise to inadvertent flashing and lamp loss under certain conditions when used together with lamps having very low breakdown voltages. A minimum breakdown voltage of about 200 is therefore desirable.
In an effort to retain the desirable fritted lead lamp construction, and at the same time elevate the low end of the breakdown voltage distribution, a number of possible lamp changes have been considered. For example, the application of a thicker frit coating raises the average breakdown as well as the maximum values found. Although the percentages of low voltage lamps is reduced, some lamps remain that break down below 100 volts. Provision of a sufficiently heavy frit coating to give a minimum breakdown voltage of 200 volts at the same time gives some lamps of such high voltage that reliability might suffer with certain camera models.
While it should be theoretically possible to exercise some control of shred contact with the primer-coated leads and thereby influence breakdown voltage, in practice this does not appear feasible. The mere shipping and handling of flashlamps causes movement and relocation of the mass of shredded combustible.
U.S. Pat. No. 4,059,388 of John W. Shaffer, describes a primer material comprising a mixture of combustible metal powder (zirconium), an additive of one of more metal oxides which are electrically non-conductive but combustion-supporting, such as WO.sub.3, and a binding agent, but which is free of oxidizer salts. The metal oxides function as an oxygen donor and do increase the breakdown voltage somewhat. The elevation of breakdown voltage attainable through such oxide addition alone, however, is insufficient to give lamp populations essentially free of lamps with less than a 200-volt minimum.
A copending application Ser. No. 744,540, Daniel W. Bricker et al, filed Nov. 24, 1976 and assigned to the present assignee, describes a primer material which includes submicron sized refractory particle additives, such as fumed silica, which are extremely fine compared to the powdered fuel particles (zirconium). This additive does not increase breakdown voltage significantly but does render the primer more sensitive to low energy discharges. Thus its use, while beneficial from the standpoint of rendering lamp reliability and ignition sensitivity independent of the zirconium powder lot used, actually increased the tendency toward inadvertent electrostatic ignition.
Another U.S. Pat. No. 3,972,673 of Schupp, describes a primer material which comprises a solid mixture of combustible fuel, an oxidizer for the fuel, such as an alkali metal chlorate or perchlorate, and a combustion supporting oxide of the type which is converted to a lower oxide upon combustion of the mixture. More particularly, the Schupp patent indicates that certain metal oxide additives in this solid primer mixture promote a more complete combustion of the primer fuel. It is hypothesized that the additive is partially reduced through chemical reaction taking place when the lamp is flashed to provide a source of oxygen which is readily available for combustion of the primer fuel by reason of the oxygen being generated in the solid mixture. The specific combustion-supporting oxides indicated as suitable for this application comprise Co.sub.3 O.sub.4, BaCrO.sub.4, Fe.sub.2 O.sub.3, and the higher oxides of nickel. A preferred primer material composition is given as comprising a solid mixture, in percentages by weight, of 46.1 percent zirconium, 14.5 percent sodium chlorate, 31.7 percent Co.sub.3 O.sub.4, and 7.7 percent BaCrO.sub.4 , and further containing between 1-5 percent of water soluble polymer binder such as polyvinyl alcohol or polyvinyl pyrrolindone. These materials are dispersed in water to provide a wet paste for manufacturing use.
A later filed, but earlier issued, patent of Schupp, namely, U.S. Pat. No. 3,969,067, describes an improvement over the primer material discussed above in that the composition further includes an alumina gel additive in an amount from about 0.25-2.0 percent by weight of the solid mixture. The patent indicates that this additive modifies the operation of the primer material to promote less sensitivity to premature accidental ignition from ambient electrostatic charges without requiring an increase in the maximum energy provided by the firing pulse. In particular, the patent indicates that the alumina gel additive can be dispersed in the primer composition with the effect of increasing the average breakdown voltage characteristic of the dried primer. A water slurry of alumina gel would be immiscible in an organic solvent mixture suitable for use with a nitrocellulose binder. Hence, this approach to the minimum breakdown voltage problem appears to be somewhat limited as to choice of additive materials and the solvent employed in providing a liquid dispersion for application to ignition structures in lamp manufacturing. Further, the amount of additive employed appears quite critical. The patent indicates that increased alumina gel concentration in the primer mixture raises the breakdown voltage level approximately 200 volts for each one percent by weight addition of the additive. Accordingly, the production control required to avoid unacceptably high breakdown voltage levels with fritted lead lamps would appear to be somewhat undesirable for low cost, high speed automated manufacturing processes.